1. Field of the Invention
The present invention relates to a plasma treatment apparatus for forming a thin film on a semiconductor substrate or a glass substrate, particularly to a radio-frequency power supply system configured to supply radio-frequency power to a discharge electrode.
2. Description of the Related Art
In recent years, to accelerate LSI device speed, copper having less electrical resistance has become more used as a metal interconnect material than aluminum. Concurrently, to reduce interconnect capacitance causing signal delay, carbon-containing silicon oxide films having low dielectric constant have become more used as an interlayer insulator material. As a method for forming carbon-containing silicon oxide films, a material having a siloxane structure and 27.12 MHz radio-frequency power are used to achieve a given deposited film structure (e.g., U.S. Patent Publication No. 2001/0046567).
13.56 MHz radio-frequency power, which has been used, promotes decomposition of a source gas having a siloxane structure and in fact decomposes the siloxane structure before a reaction gas (i.e., a mixture of the source gas and various additive or other gases) reaches a semiconductor substrate. In contrast, 27.12 MHz radio-frequency power enables carbon-containing silicon oxide films to deposit on the semiconductor substrate without destroying the siloxane structure of the source gas.
Additionally, as barrier films for preventing copper diffusion, silicon nitride films having a dielectric constant of approximately 7 had been used. Recently, silicon carbide films having a dielectric constant of approximately 4 to 5 have begun to be used. To form the silicon carbide films, an alkylsilicon compound having intramolecular silicon-carbon bonds is used as a source gas. If reaction gas decomposition is promoted and becomes excessive, hydrogen atoms are discharged from alkyl groups in reaction gas molecules and are taken into the film. In the result, leakage current in silicon carbide films reaches a high level of 1×10−7 A/cm2 (when an electric field applied to the silicon carbide films for measuring leakage current is set at 2 MV/cm), causing an electrical current to leak into a metal interconnection. To lower a leakage current level of silicon carbide films, using relatively high 27.12 MHz radio-frequency power is effective for inhibiting hydrogen atoms in alkyl groups contained in the reaction gas from dissociating therefrom.
Furthermore, to reduce production costs of LSI devices, use of 300 mm diameter semiconductor substrates has begun in recent years. Based on this trend, uniformity of large-area treatment has become a requirement; particularly for a parallel-plate-type plasma treatment apparatus which treats one or multiple semiconductor substrates in a reactor (e.g. a plasma CVD apparatus, and a plasma etching apparatus), uniform temperature distribution and uniform plasma formation between electrodes are required throughout a semiconductor substrate surface. In large-area parallel-plate-type electrodes, which use radio-frequency power of 27.12 MHz or higher, electric field distribution becomes non-uniform due to voltage distribution affected by standing waves (e.g. “Longitudinal voltage distribution in transverse rf discharge waveguide lasers” by D. He et al., J. Appl. Phys. 54(8), August. 1983, p.4367).
This non-uniform electric field distribution causes non-uniform growth rates of a film formed on a semiconductor substrate, making it impossible to obtain a film having a uniform thickness on the entire surface. To solve this problem, a method of applying radio-frequency power equally to each of multiple power supply points of a discharge electrode by distributing the radio-frequency power using multiple coaxial cables having equal length and equal characteristic impedance was reported (e.g. JP Patent No. 3332857, U.S. Pat. No. 6,353,201). Using this method, film thickness non-uniformity of ±10% or less has been achieved.
Film thickness non-uniformity required for a manufacturing apparatus for recent LSI devices is ±3% or less for 300 mm diameter semiconductor substrates. Even if an impedance matching box and a discharge electrode are connected using coaxial cables having equal length and equal characteristic impedance, characteristic impedance of each radio-frequency channel does not become practically uniform due to stray capacitance arising between the radio-frequency channels and their peripheral parts and/or individual characteristic-impedance differences caused by characteristic errors of the coaxial cables. Additionally, individual differences of losses also arise at each connection of a coaxial cable and a power supply point. As a result, the electric field becomes non-uniform and film thickness non-uniformity becomes approximately ±10%, and thus film thickness non-uniformity within ±3%, which is required for recent LSI devices, cannot be achieved.